First evidence of the trisulfur radical ion S3- and other sulfur polymers in natural fluid inclusions

Sulfur plays a key role in numerous processes occurring in the Earth's crust. However, its speciation in deep and hot geological fluids remains poorly constrained. Here, we used quantitative in-situ Raman spectroscopy on natural fluid inclusions from deep sedimentary environments where thermochemical sulfate reduction (TSR) occurred to determine the sulfur speciation at temperatures representative of their entrapment conditions (100-300 degrees C). Results unambiguously demonstrate the presence of the trisulfur ion S-3(-) and other polymeric S species (Sn2- +/- S-n(0)) at temperature (T) > 100 degrees C, whereas only sulfide and sulfate were detected at 25 degrees C. From 200 to 300 C, sulfate and sulfide, the two dominant S species, Contribute to 41 9% and 59 9% of the mean total dissolved S concentration aStot] = 0.25 mol/kgH(2)O = 0.8 wt%), respectively. The S3 concentration accounts for 0.2 to 3% of Stot in this T range, with a maximum recorded concentration of 2.9 x 10 2 mol/kgH(2)O (2780 ppm) at 300 degrees C. This observation implies that the TSR process occurs under physico-chemical conditions that enhanced the stability of S-3 and other polymeric S species. This conclusion has important consequences for the genesis of base metal sulfide deposits and sour gas fields where reduced sulfur originates from TSR.